In the formation of a semiconductor chip, integrated circuit devices, such as transistors are first formed at the surface of a semiconductor substrate in the chip. Interconnect structures are then formed over the integrated circuit devices. Bumps are formed on the surface of the chip, so that the integrated circuit devices can be accessed.
In a typical bump formation process, an under-bump metallurgy (UBM) is formed, followed by the formation of a bump on the UBM. The UBM formation may include forming a copper seed layer and forming and patterning a mask on the copper seed layer so that a portion of the copper seed layer is exposed through an opening in the mask. A plating step is then performed to plate a thick copper layer on the exposed portion of the copper seed layer.
In the forming and patterning of the mask, residues (known as scum) of the mask may be undesirably left or be generated as a by-product of the patterning step. A descum step is then performed to remove the scum before the copper plating. It was observed that the queue time (Q-time) after the descum step was often very short, sometime shorter than 12 hours, wherein the Q-time is the time that the respective wafer can be stored without incurring significant degradation before the copper plating is performed. However, many process steps may be needed before the copper plating is performed, and these process steps may take a long period of time. If the Q-time expires before the copper plating has been performed, the respective wafer has to be re-descumed to re-clean the surface of the wafer. However, the re-descum would cause a change in the shape and dimension of the mask, and hence a change in the shape and dimension of the resulting plated copper.
Another problem found in the conventional bump formation process is the bump-lost problem. During the subsequent bonding process, bumps may peel off due to an unreliable adhesion between the copper seed layer and the plated copper. The above-discussed challenges contribute significantly to the high manufacturing costs and poor bump reliability.